Nagoya 200C 155/460 antenna: Radials or not? A backyard test.

[fremont]

This antenna is 1/2 wavelength at 155 MURS and 5/8 at 460 (e.g., GMRS).  I am going to use it as a base antenna on top of a NMO mount.  For GMRS, it'll primarily be used for simplex communications, but there will be instances whereby it'll go through repeaters.

I had a couple of questions--which I subsequently tested:  1) Would the GMRS (5/8 wave) frequencies benefit from elevated radials and, if "yes," then 2) how long should they be?  Question #1 gets debated a fair amount from what my cursory review of the interweb uncovered.....it seemed as the antenna would benefit from some type of radial.  To answer Q #2, I went to this website to get the radial length.  (By entering 465.200--halfway between GMRS duplex Tx and Rx frequencies, I was given the answer of 6" radials.  I cut mine from 1/8" alum. rod.)

The results on MURS (155)--again, a 1/2 wave antenna at 155--were consistent:  The commercial (Blue4, Green5) channels near 155 performed better than Ch 1-3 near 152, with SWRs of 1.35 and 2.19, respectively.

For UHF, including GMRS, the results were interesting.  By adding the radials, the SWR graph became the traditional very steep parabola which helped the UHF frequencies especially near repeater inputs, e.g., 440 (3.05), 445 (2.35), 450 (1.84), 462.800 (1.55), 465.200 (2.06) and 467.600 (2.68).  Without radials, it was a mildly meandering, relatively flat but downward sloping line, e.g., 440 (2.98), 445 (2.54), 450 (2.36), 462.800 (1.95), 467.200 (1.59), 467.600 (1.64).

So, I concluded that I would use the radials, since a) I want to use the antenna to hit both FM and DMR UHF repeaters and b) the majority of my GMRS QSOs will be via simplex.  The antenna is definitely tuned for what it says on the label:  155 and 460.  Not 155 and 465.

Does this sound right to the RF engineers of the world?

[KAF6045]

If using enough radials, you could try cutting half for 467MHz and the other half for 462MHz.

A halfwave should not require radials regarding SWR, but could have the radiation pattern affected by the presence of radials (the radials may appear as a ground close to the base of the halfwave, rather than the real ground some feet below -- the likelihood is that it would cause the main beam to be at a higher elevation).

[fremont]

3 hours ago, KAF6045 said:

If using enough radials, you could try cutting half for 467MHz and the other half for 462MHz.

Total of four and there's only 1/8" difference between 462 and 467.  I've bent the radials to a 40 deg angle in an attempt to best match impedance.  Feels like splitting hairs at this point, however.

[KAF6045]

10 hours ago, fremont said:

Total of four and there's only 1/8" difference between 462 and 467.  I've bent the radials to a 40 deg angle in an attempt to best match impedance.  Feels like splitting hairs at this point, however.

I'd need to study to figure out how to represent a 5/8 wave in EZ-NEC. As a simple wire with four (not-tilted) radials, I'm getting an SWR of 25:1. (Also need to check where EZ-NEC is getting the description from; it is not in free space, and not a dipole)

Plotted for an antenna at 10m elevation.

I tried running for MURS frequencies, but it shows SWR >100:1

 

[SteveShannon]

I’m sure you realize this, but you must have a loading coil to match to a 5/8 wave antenna.

Here’s a pretty concise (albeit with some sloppy spelling) article about 5/8 wave antennas:

https://www.cainetworks.com/products/antenna/

 

But here’s an even better article:

https://practicalantennas.com/designs/verticals/5eights/

[Lscott]

3 hours ago, KAF6045 said:

I'd need to study to figure out how to represent a 5/8 wave in EZ-NEC. As a simple wire with four (not-tilted) radials, I'm getting an SWR of 25:1. (Also need to check where EZ-NEC is getting the description from; it is not in free space, and not a dipole)

Plotted for an antenna at 10m elevation.

I tried running for MURS frequencies, but it shows SWR >100:1

 

Try this model file.

Five Eights Wave Rev 5.EZ

[KAF6045]

23 hours ago, Lscott said:

Try this model file.

Five Eights Wave Rev 5.EZ 1.48 kB · 1 download

Well, after rescaling to GMRS from 2m... And reducing segment count... putting in real ground, and raising the antenna to ~20 feet

SWR looks better... (hmm, maybe even better with the higher segmentation of the original file -- likely because the feed/load moved with fewer segments -- yes, going back to 90 segments did put SWR at design frequency)

OUCH!

Rather high take-off angle. Beam toward horizon is 1.7dBi, vs 5.03dBi at 37 degrees above horizon. Might be great if at the bottom of a valley hitting mountain-top repeaters The horizon is seeing half the power seen at 37deg.

 

Still not usable as MURS - as modeled - with a 25:1 SWR, but it does have a nicer pattern for reaching the horizon.

 

Moving it up another 10 feet (approximate match to original plots) changed the max GMRS to 40 deg, 5.34dBi, horizon to 2.44dBi (only -2.9dB from max -- still, practically half the power)

[fremont]

1 hour ago, KAF6045 said:

Still not usable as MURS - as modeled - with a 25:1 SWR, but it does have a nicer pattern for reaching the horizon.

 

Interesting work-up.  155 MURS is 1/2 wave for this antenna.  I'm getting <1.5 SWR at the commercial Blue/Green MURS frequencies with 6" 90 deg radials.

[Lscott]

Antenna modeling takes some intuitive insight. Also have to get a feel for the limitations of the modeling tool(s) as well.

The high segment count I used was to keep the feed point and series inductive load located at the bottom of the main vertical element. The 5/8 wave antenna requires a series inductive element to match correctly.

The dual band antenna I haven't tried to model yet. One reason is you need multiple elements, at the higher frequency, in series using phasing elements or line sections to get the element currents to line up phase wise. Trying to figure out how to do that with a modeling program will take time and good educated guesses on how to do that and get believable results.   

[Lscott]

This is another model file for a 5/8 wave MURS antenna. In this case there are 4 horizontal ground radials. Beside a series inductive element I need to use a 1/4 transmission line matching section to bring the SWR down to near 1:1 at the design center frequency, that's between the 151 MHz and 154 MHz channel frequencies. Without the transmission line matching section the match isn't that great. Looks like the radials really should be angled downward.

I also tried a J-Pole model at MURS too.

MURS J-Pole Rev 2.EZ MURS Five-Eights Wave GP - Rev 3.EZ

[tcp2525]

On 10/25/2022 at 12:26 PM, Sshannon said:

I’m sure you realize this, but you must have a loading coil to match to a 5/8 wave antenna.

Here’s a pretty concise (albeit with some sloppy spelling) article about 5/8 wave antennas:

https://www.cainetworks.com/products/antenna/

 

But here’s an even better article:

https://practicalantennas.com/designs/verticals/5eights/

I've used a matching loop like what you find on the Cushcraft Ringo antennas with excellent  results on every 5/8 wave I built. Extremely easy to build and tune.